CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance), which adopts a wireless LAN (Local Area Network) based on the IEEE802.11 standard as a wireless communication system communicating between a wireless access point and a wireless terminal, is widely known. IEEE Std 802.11™-2012 and IEEE Std 802.11ac™-2013 have been known as specifications of wireless LAN. The IEEE802.11ac standard adopts downlink multiuser MIMO (DL-MU-MIMO) technology that extends MIMO (Multi-Input Multi-Output) technology. In the downlink multiuser MIMO, an access point uses a technology called beam forming to be capable of data transmission to individual wireless terminals by way of beams spatially orthogonal to each other, allowing different data to be simultaneously transmitted to a plurality of wireless terminals. This makes it possible to improve a system throughput.
Additionally, in order to attain a further high-efficiency, a Study Group (SG) called IEEE802.11 HEW (High Efficiency WLAN) for investigating a standard successive to the IEEE802.11ac standard was approved in May, 2015, where a technology for aiming at the high-efficiency has been investigated. One of technology candidates for that includes an uplink multiuser MIMO (UL-MU-MIMO) technology. In the uplink multiuser MIMO, a plurality of wireless terminals perform data transmission at the same timing to an access point by way of beams spatially orthogonal to each other, giving high-efficiency of the uplink transmission.
In order to sufficiently obtain effects of spatial multiplexing by the multiuser MIMO, a user multiplexing number is preferably maintained above a certain value even in retransmitting the data. For example, assume that in a new transmission, spatial multiplexing transmission is performed by four wireless terminals, where a CRC (Cyclic Redundancy Check) error occurs only in any of the data. In this case, if the data only is retransmitted, as a result, user multiplex is not performed and the system throughput decreases.
There has been known, as a method resolving that, when the access point performs retransmission in downlink multiuser MIMO transmission, new data is newly multiplexed in addition to retransmission data to improve a usage efficiency. This allows the user multiplexing number above a certain value to be maintained even in the retransmission. Therefore, even if the retransmission occurs, the system throughput can be improved owing to the spatial multiplexing.
However, this method is specialized for the downlink multiuser MIMO without taking account of the uplink multiuser MIMO. In other words, in a case of the downlink multiuser MIMO transmission, the access point can perform the transmission in combination of the retransmission data and the new data based on a determination by the access point on the basis of an acknowledgement response result sent as a reply from each wireless terminal.
On the other hand, in a case of the uplink multiuser MIMO transmission, individual wireless terminals are multiuser MIMO transmitting devices, which means that a plurality of transmission terminals exist. For this reason, a wireless terminal wanting to transmit the new data cannot determine whether to simultaneously multiplex the new data to transmit at the same time as a wireless terminal transmitting the retransmission data. Further, in the uplink multiuser MIMO, there is no scheme for multiplexing and transmitting the retransmission data and the new data by a plurality of wireless terminals. Therefore, if the data retransmission occurs, the high-efficiency uplink multiuser MIMO transmission maintaining the user multiplexing number above a certain value cannot be achieved.